When an oil or gas well is no longer economical, or if there is some problem with the well which means that production is no longer possible or that well integrity has been compromised in some way, or for other reasons, the well may be abandoned. It is common practice to plug the well before abandoning it, e.g., to prevent seepage of hydrocarbon product or water from the well. This can also apply to water injectors, i.e. bores which have been drilled in order to pump water into a reservoir to increase bottom hole pressure.
Commonly, plugging may be achieved by injecting a settable substance or medium, e.g., cement, into the well. A well will normally have production perforations, that is to say apertures in a well liner or casing through which hydrocarbon product enters from the rock formation and travels to the surface. During plug and abandonment operations it is common to seal (“squeeze”) production perforations with cement or another settable medium which may then form a permanent barrier to flow across the perforations and out of the well.
The well to be plugged and abandoned may in some cases have production screens or be gravel packed.
The plugging process often involves pumping a surfactant liquid, known as a “spacer”, into the well. The purpose of the spacer is to remove oil residues from the internal surface of the well casing and/or liner and rock matrix making them “water wet” (allowing better adhesion by cement). Commonly, immediately following the spacer, cement is pumped down the well to occupy the part of the well casing and/or liner where perforations are to be squeezed. The cement may also be used to seal off production screens or gravel packed completions. When sufficient cement has been pumped down, more spacer liquid and possibly other liquids may be pumped down the well in order to place the cement at its final designed location.
It is desirable to be able to monitor with a reasonable degree of accuracy where the different constituents of the liquid column are located at any given time and the associated surface pumping (treating) pressure. It is also desirable to be able to control the progress of the liquid column, and other materials being displaced, by varying pressure on the column applied at the surface. For these things to be achieved, it is helpful to have a continuous column of liquid being pumped into the well.
It is therefore desirable to have sufficient reservoir pressure entering the well (“bottom hole pressure”) to support a standing column of relatively high specific gravity material, e.g., spacer liquid, cement and displacement fluid, reaching to the top of the well. The spacer and cement and other liquids may then be pumped down against this pressure and thereby an accurate determination of each constituent's location can be determined at any given point within the process.
In many cases, the bottom hole pressure is insufficient to support a standing column of fluid reaching the wellhead at surface. In this event, positive pressure against the fluid column at the surface cannot be maintained as liquids are introduced into the top of the well. This results in liquid free falling down the wellbore and out through the reservoir completion, i.e. the perforated section of casing/liner or screen.
In this situation, it is often not possible to monitor when the cement has reached the desired wellbore location with respect to placement for sealing the wellbore. Without an accurate understanding of where the cement is, it is possible to over-displace the cement by continuing to introduce fluid at the surface which freefalls and over displaces the cement, with the result that the proximal perforations or screens not being effectively squeezed. Alternatively, it is possible to under-displace the cement thereby leaving distal perforations or screens unplugged and at the same time creating a barrier in the more proximal part of the liner hindering further optimized plugging operations within the wellbore without revision to procedures.
In the past, attempts have been made to address these issues by the addition of solid plugging material to the liquid plugging fluid or by displacement of the liquid plugging fluid with a low specific gravity fluid.
The addition of solid plugging material partially closes off pathways at the perforations, screens or gravel pack creating backpressure or the need for additional pumping pressure at the well surface in order to displace the plugging to the desired location. Thus a positive pressure on the fluid column is maintained at the well surface. Partly closing perforations with solid material can be undesirable since the perforations can end up not adequately sealed. Conversely, because there is little control over the degree of plugging and at which point the plugging will occur, an undesirable outcome can result if all perforations or screens are plugged off with solid medium prior to achieving the desired designed displacement.
Similarly, displacement of the plugging fluid with a significantly lower specific gravity fluid may also allow positive pressure to be maintained at the well surface. However, there are a limited number of available low specific gravity fluids and the constituents incorporated in them that meet design requirements.